1. Field of the Invention
This invention relates to color imaging devices. More particularly, it relates to solid-state photosensitive devices that have a planar array of charge-handling semiconductive photosensors in micro-registration with a multicolor planar array of filter elements, and to methods of making them. The color imaging devices of the present invention are particularly useful in solid-state video cameras.
2. Description Relative to the Prior Art
A reliable, yet sensitive, all solid-state video camera would find abundant utility, including use in television cameras, card readers, facsimile recorders, picturephones, character recognition, etc. Solid-state video cameras would be desirable because, in addition to the problems inherent in traditional video cameras of drift, misalignment and short tube life, such traditional, i.e., non-solid-state, video cameras suffer from the complications of registering separate electron beams and the effects of electron beam lag. A relatively simple, efficient solid-state color camera which would overcome these problems is still sought.
Color photosensitive devices using charge-handling solid-state image sensors of various types, for example, charge-coupled devices, known as CCDs, and charge-coupled imagers known as CCIs, have been proposed for and used in video cameras. To avoid optical complexity and problems with image registration, it is highly desirable that color image sensing occur at a single imaging site, e.g., at a single planar photosensitive array. Many problems are encountered with such "single-site" color imaging, however, because at least three distinct types of color information must be extracted in order to represent a color image in video signal form.
Some of the problems associated with "single-site" color imaging processes are overcome by the approach taken in U.S. Pat. No. 3,971,065, issued July 20, 1976, in the name of B. E. Bayer. In the Bayer approach, color imaging is effected by a single imaging array composed of a large number of individual luminance and chrominance sensing elements that are distributed according to type (sensitivity) in repeating interlaid patterns, wherein the luminance pattern exhibits the highest frequency of occurrence--and therefore the highest frequency of image sampling--irrespective of direction across the array.
To produce an element array according to the Bayer approach or other similar approaches, a solid-state sensor array wherein each sensor has a broad wavelength sensitivity is provided with a superposed filter array. Methods for producing multicolor filter arrays for various purposes are known in the art; however, many of these methods are not adaptable for producing color filter arrays which are useful with a solid-state sensor array. For example, multicolor filter arrays that resort to the use of multiple layers are not desirable for single-site color imaging devices, because such arrays require the imaging optics to have a large depth of field so that all layers, as well as the photosensors, are in focus. Further, multilayer arrays can result in misalignment between the individual filter elements and the underlying photosensors.
One method for providing a single layer multicolor filter array is described in commonly assigned copending U.S. Patent Application Ser. No. 730,885, filed Oct. 8, 1976, now abandoned, continuation-in-part application Ser. No. 867,841 filed Jan. 9, 1978 now U.S. Pat. No. 4,204,866. In this application, the color filter array is formed in a dye mordant layer. Dyes are imbibed from a solution into the mordant layer in window patterns using photoresist techniques. Another method for providing a single layer multicolor filter array is described in commonly assigned copending U.S. Application Ser. No. 730,886, also filed Oct. 8, 1976, now U.S. Pat. No. 4,081,277. In the method of this application, heat-transfer dyes are transferred into a dye-receiving layer. Again, photoresist techniques are used to form window areas through which dyes are transferred into the dye-receiving layer. While both of these processes result in filter elements having excellent properties, they both involve repeated application, exposure and removal of photoresist. The use of photoresist can complicate the process of making the filter array and it can result in non-uniform filter elements. For example, non-uniform processing of the photoresist can result in window areas in one part of the array not being as cleanly washed out as window areas in another part of the array. This, in turn, can result in a non-uniform density for the filter elements formed through these windows. It would be highly desirable if these photoresist-related steps could be eliminated.
While it is fairly easy to enumerate the criteria that must be met by a successful color filter array for a color imaging device, it has proven extremely difficult to find an inexpensive process which will result in a color filter array which meets all of these criteria. Thus, while it is known that the color filter array must be formed in a relatively thin layer, that the elements within the array must have excellent edge sharpness, that the density of each individual filter element in the array must be sufficient to adequately control the sensitivity of the underlying photosensor, etc., it is extremely difficult to meet all of these criteria. This is due, in part, because properties meeting some of these criteria are incompatible for meeting others. For example, for a given dye concentration, the thinner the layer the lesser the density. It is difficult, therefore, to get a layer which is, at the same time, thin enough to avoid depth of field problems in the imaging optics and capable of providing sufficient dye density for good color separation in the imaging system. In the past, it has been possible to produce color filter arrays having the desired physical properties, but only by using expensive photoresist techniques. Thus, a color filter array and a method for making it which has all of these properties is still actively sought.